1. Field of the Invention
The present invention pertains to carbon supported catalyst compositions for fuel cells that are characterized by good stability, durability, and high activity. In particular, it relates to carbon supported catalyst compositions for use in solid polymer electrolyte fuel cells and to methods for making same.
2. Description of the Related Art
Solid polymer electrolyte fuel cells convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. Such fuel cells generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). Each electrode in a MEA comprises an appropriate catalyst composition and optionally a fluid diffusion layer that are arranged such that the catalyst composition is located between the polymer membrane and the fluid diffusion layer. Commonly, the catalyst compositions comprise a noble metal catalyst (e.g. Pt) that is supported on a high surface area support (e.g. activated carbon) in order to obtain high electrochemical activity. For gaseous reactants, the fluid diffusion layer is known as a gas diffusion layer and is typically made of porous carbonaceous material (e.g. carbon fiber paper). The gas diffusion layer serves to distribute reactants to and by-products from the catalyst composition, as well as to provide mechanical support and electrical connection to the catalyst composition. MEA stability and durability, and particularly that of the catalyst compositions, are some of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive applications for instance, an MEA may be required to demonstrate durability of about 6,000 hours. However during fuel cell operation, the platinum in conventional catalyst compositions can dissolve, re-deposit in the membrane, and agglomerate, resulting in electrochemical surface area losses, and an associated decrease in catalyst activity. And in supported catalyst compositions, there can be stability issues associated with either the catalyst or the support or both. For instance, corrosion of the carbon support is a known concern in carbon supported Pt catalyst compositions. The fuel cell industry, especially the automotive fuel cell industry, demands active and stable cathode catalysts.
Significant research into catalysts for such fuel cells has been ongoing. For example, Miguel Cruz Quinones at the Cornell Center for Materials Research (Characterization of ordered intermetallics as catalysts for fuel cell Applications, 2005) investigated new intermetallic compounds for fuel cell anodes using cyclic voltammetry. Among the most promising ordered intermetallic phases were Pt3Ta, Pt2Ta, Pt3Ti and Pt3Nb.
D. C Papageorgopoulos et al. investigated the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant anodes for solid polymer electrolyte fuel cells (Electrochimica Acta 48 (2002) 197-204). Here, the noble metals were present in highly dispersed form but without being alloyed to each other.
Others have also investigated improved catalysts for fuel cell electrodes. In WO2009/090125, an improved electrode catalyst was disclosed comprising platinum or platinum alloy and a metal oxide such as tantalum oxide and/or niobium oxide. In US2010068591, fuel cell electrodes comprising Nb2O5/Pt/C catalyst were disclosed.
While certain Pt alloys show promise for use in fuel cells, it is difficult to form such alloys without using very high synthesis temperatures. But highly dispersed, high surface area catalyst is desired for purposes of providing high activity per unit mass and exposure to high temperatures generally tends to cause Pt catalysts to coalesce with a substantial reduction in surface area and hence activity per unit weight. Recently, in patent application WO2011/038907, catalyst compositions were disclosed comprising intermetallic PtMe on a Nb or Ta oxide support. These compositions showed improved stability and durability over Pt. The compositions were made by heating a Pt compound, an appropriate Nb or Ta salt, and a basic salt at low temperature (e.g. <900° C.). Dispersions and supported catalyst compositions were not prepared.
There remains a continuing need for more stable and durable catalysts for a fuel cell electrode is needed, and especially one that can be easily synthesized. The present invention addresses these and other needs.